The present invention relates to a paint composition and a sliding part.
Since fluoro-resins excel in heat resistance, sliding properties, and water and oil repellency, a resin layer containing fluoro-resin powder has conventionally been formed on the surface of a sliding part to improve the sliding properties. For example, Japanese Laid-Open Patent Publication No. 59-108850 proposes a piston for an engine whose outer surface of the skirt portion is baked-coated with a coating material prepared by dispersing flaked aluminum and fluoro-resin powder in a heat-resistant resin (epoxy resin). Japanese Laid-Open Patent Publication No. 10-122040 proposes a piston for an internal combustion engine wherein a coating layer composed of fluoro-resin, polyimide, and iron oxide powder is formed on the peripheral surface of the piston skirt. Japanese Laid-Open Patent Publication No. 2001-11372 proposes a paint composition composed of polytetrafluoroethylene (PTFE), polyamide imide, and alumina particles, wherein the weight ratio of PTFE to polyamide imide is 5 to 10 inclusive: 95 to 90 inclusive, and containing 5 to 10 parts by weight inclusive of alumina particles to 100 parts by weight of the total of PTFE and polyamide imide. A piston for an internal combustion engine that has the coating film of the paint composition on the peripheral surface of the piston skirt is also proposed. Japanese Laid-Open Patent Publication No. 2000-249063 proposes the formation of a coating layer containing fluoro-resin as the major component on the peripheral surface of the piston of a compressor used in the air-conditioning system of vehicles.
The lubricant oil in a compressor is atomized by compressing coolant gas, such as chlorofluorocarbon flowing when the compressor is operated, which is then delivered to the sliding parts composing the internal mechanisms of the compressor to lubricate the sliding parts. However, when a compressor kept without operation for a long period of time is restarted, the lubricant oil adhered on sliding portions may be flushed away by the coolant gas. For example, in the initial stage of the operation of a swash plate compressor, wherein a piston is connected to the swash plate via a shoe, and is reciprocated in a cylinder bore by the rotation or oscillation of the swash plate, the swash plate slides on the shoe before the lubricant oil reaches the sliding surfaces of the swash plate and the shoe. Moreover, before the lubricant oil reaches the sliding surfaces of the swash plate and the shoe, the coolant in the gaseous state reaches the sliding surfaces, and plays a role to flush away the lubricant oil remaining on the sliding surfaces. Therefore, in the initial stage of the operation, the swash plate slides on the shoe in a dry state without the lubricant oil. During the period after the compressor is started and before the coolant gas returns to the compressor to form an oil mist (the period is only about one minute), a sufficient quantity of oil is not present on the sliding portions that require lubrication despite the operation of the compressor. It is extremely important to secure lubrication on the sliding portions for such a period when the quantity of lubricant oil is insufficient.
In recent years, carbon dioxide has attracted people's attention as a coolant for compressors; however, when carbon dioxide is used as a coolant, the compressive load on a swash plate via a piston is significantly larger than the compressive load when a chlorofluorocarbon-based coolant is used. In order to secure high reliability for compressors even in such a strict sliding environment, the improvement of sliding properties of the sliding portions, through the improvement of seizure resistance, wear resistance, and adhesion to base materials of the sliding portions, has been demanded.
As described above, the improvement of sliding properties by forming a coating layer containing fluoro-resin powder on the surface of a sliding part is disclosed in the above-cited four patent publications. Although fluoro-resins excel in heat resistance, chemical resistance, weather resistance, sliding properties, water and oil repellency, and electrical properties, the integration of fluoro-resins with other materials is difficult because of the difficulty of adhesion. The members composed of a fluoro-resin also has poor adhesiveness to the adhesives used for cladding the member composed of a fluoro-resin with other members.
Among the above-cited four patent publications, Japanese Laid-Open Patent Publication No. 59-108850, Japanese Laid-Open Patent Publication No. 10-122040, and Japanese Laid-Open Patent Publication No. 2001-11372 disclose that the average particle diameter of the fluoro-resin powder is made within a predetermined range for improving the stability of dispersion in paint, and the flatness of formed coating films. However, none of these patent publications disclose the improvement of the sliding properties of resin layers containing fluoro-resin powder by improving the wettability and adhesiveness of the fluoro-resin particles.